dolomiah wrote:
I have a website on hackaday with various pictures, hope that gives you an impression of what I have done.
The image above shows that these form a loop, with the signal wiring in yellow on the bottom and left of the loop, and the return wiring in black along the top and right. (For clarity I'll ignore the red Vcc wires, but they too act as a return path.)The signal wiring offers little scope for improvement. It's long, and not much can be done about that. But can the return wiring be improved? Yes, and one improvement is to add a diagonal directly between the two chips. But, interestingly and counterintuitively, that's still not the ideal path -- the route the return current most wants to take. What it wants most of all is to hug the signal path, even though that's *longer* than a direct route (ie, diagonal)!
So, I'd consider adding a ground wire that takes the same, long-ish route as the yellow signal path. (This new ground wire needs to be connected at both ends; otherwise it isn't a path.)Of course the CPU and RAM aren't the only chips, and signal connections go every which way, helter skelter. All these signals paths should have prospective return paths available, but that'd result in a complete rat's nest. A ground plane would solve the problem -- because it attaches every point with every other point -- but we can't do that, either.
More practical (though much less satisfactory) than a plane is to provide a coarse grid of return connections. The idea of the grid is to help ensure that any two arbitrary points will have a return path close by. The grid needn't be perfectly orderly. But, operating on the premise that any two arbitrary points may have a signal path between them, we want a return path reasonably close by. As an example, consider C and D in the image below. I would add a horizontal path from C to D, and also extend the path to meet the columns to the left and right. I'd also put a return path from A to B. Right now there's no chip at A, but presumably there will be one in future, and it might have a signal path to B. And right now the only return path to B involves a drastic detour. I hope I've managed to write this in a way that makes some kind of sense. If there are some edits later it won't be surprising.
Cheers!
In the future, I'll say the address becomes valid sometime after the fall of Ø2, which will be almost as unhelpful as saying "roughly midway." 
